In the production of petroleum and gas, corrosion caused by carbon dioxide, which is known as carbonic corrosion, takes place at the bottom of wells, where the temperature is generally about 60xc2x0 C. or more, in the surface pipelines and in refining of the crude oil. The approach usually adopted to overcome this is to use corrosion inhibitors based on amine salts, quaternary ammonium salts, imidazolines or phosphoric esters, but these compounds are not entirely satisfactory since they are ecotoxic in marine environments (acute toxicity to Skeletonema costatum at less than 1 ppm).
In European patent applications EP-A-520,761 and EP-A-526,251, it is mentioned that it is possible to reduce the toxicity of a diamine or of an imidazoline by reacting these molecules with acrylic acids; the larger the number of acrylic acid molecules, the more the toxicity reduces.
European patent EP-A-567,212 is based on the low toxicity of morpholine salts and phosphoric esters.
It has now been found that good inhibition of the carbonic corrosion of iron may be obtained with non-ecotoxic compounds of water-soluble polymethylenepolyaminodipropionamide type having a very low marine ecotoxicity (50 to 100 ppm on Skeletonema costatum), the general formula of which compounds is:
H2NCOxe2x80x94(CH2)2xe2x80x94NHxe2x80x94[xe2x80x94(CH2)nxe2x80x94NHxe2x80x94]mxe2x80x94(CH2)2xe2x80x94CONH2
in which m is an integer which may take any value from 1 to 4, which represents the number of polymethyleneamino xe2x80x94(CH2)nxe2x80x94NHxe2x80x94 chain units, it being possible for n to have, in each polymethyleneamino chain unit, an integer value from 2 to 6. This process for limiting the carbonic corrosion of steel in aqueous media forms the basis of the present invention, this process consisting in using, as non-ecotoxic corrosion inhibitor, an aqueous composition containing as active material at least one of the said polymethylenepolyaminodipropionamides, with the exception of tripropylenetetraaminedipropionamide. These compounds may be taken individually or as a mixture, and moreover the corresponding technical products are themselves mixtures of such individual compounds.
These polymethylenepolyaminodipropionamides are readily obtained by condensation of acrylamide with polymethylene-polyamine bases, for example the EDA, DETA, TETA, TEPA and HEPA bases in the ethyleneimine series, the PDA and DPTA (norspermidine) bases in the propyleneimine series, or hybrid compounds, for example of the spermine H2Nxe2x80x94(CH2)3xe2x80x94NHxe2x80x94(CH2)4xe2x80x94NHxe2x80x94(CH2)3xe2x80x94NH2, or spermidine H2Nxe2x80x94(CH2)4xe2x80x94NHxe2x80x94(CH2)3xe2x80x94NH2 type.
They are fully characterized by NMR spectrography (see Table I later).
These compounds may be used both in continuous injection and in a batchwise treatment. The useful doses are between 5 ppm and 20 ppm. They may be used directly or in the form of an aqueous or aqueous-alcoholic solution, preferably an aqueous-glycolic solution for flash-point reasons, containing from 20% to 65% by weight.